The use of a combination of a gas turbine and a steam turbine for power generation, has the advantage that it is more efficient than power generation with either type of turbine. The combination will hereinafter also be referred to as "combined cycle". A difficulty of lubricating a combined cycle resides in the large variety of different components which must be lubricated, i.e. bearings (both journal and thrust), gears, hydraulic control systems, flexible couplings and oil shaft seals. Although each component could be lubricated per se, it is advantageous to have a single, common lubricating system containing a single, common lubricant for all components.
The lubricating composition for use in the common system will have to meet a rather outstanding combination of requirements in order to satisfactorily lubricate each component. These requirements comprise the rather severe thermal and oxidative stability requirements and stringent foaming levels for the gas turbine part, whereas the steam turbine part requires the oil to have good water shedding properties and excellent corrosion resistance. High temperature oxidative stability means that the lubricating composition has a low tendency to form sludge, a low increase in viscosity and a low increase in total acid number at high temperature. Desirable characteristics are a viscosity increase of at most 20%, a total acid number increase of at most 3.0 mg KOH/g and a sludge content of less than 300 mg/100 ml after having been subjected to the oxidation stability test DIN 51394 performed according to the high temperature modifications set out in the General Electric specifications GEK 32568 C and GEK 101941, preferably less than 250 ml/g, more preferably less than 200 ml/g, most preferably less than 150 ml/g. In establishing the amount of sludge produced, the sludge must be removed carefully from all the equipment used in the test. The sludge is separated from the oil by filtration.
It has been found that especially the tendency of the lubricating composition to form sludge, is important in whether a lubricating composition is suitable for use in lubricating combined cycle equipment.
The lubricating composition for use in combined cycle must further have a certain viscosity index, pour point, filterability and anti-wear performance, while the composition should provide adequate lubrication over many years.
Achieving this combination of complex lubricant properties allows the lubricating oil to be suitable for use in a combined cycle.
In EP-A-696 636 it is taught to use a combination of (A) a lubricating base oil, (B) an alkyl diphenylamine and/or phenyl-.alpha.-naphthylamine and (C) oxymolybdenum sulfide dithiocarbamate and/or oxymolybdenum sulfide organophosphorodithioate. One kind of alkyldiphenylamine may be used alone, or two or more kinds of alkyldiphenylamine may be used together. One kind of phenyl-.alpha.-naphthylamine may be used alone or two or more kinds of phenyl-.alpha.-naphthylamine may be used together. Also one or more kinds of alkyldiphenylamine and one or more kinds of phenyl-.alpha.-naphthylamine may be used together. In the examples, either a single alkyl diphenylamine or a single phenyl-.alpha.-naphthylamine is used. Oxymolybdenum sulfide organophosphorodithioate is present in only one of the examples according to the teaching. EP-A-696 636 does not disclose or teach to use the specific combination of alkyl diphenylamine and phenyl-.alpha.-naphthylamine and thiophosphate.
Further, it is known to use a combination of amine antioxidants, optionally in combination with a thiophosphate, in ester base oils for use in gas turbine engines for aviation purposes. Such formulations have been described in GB-B-990,097, GB-A-2272000 and GB-B-1,293,245. These documents do not disclose or teach to use a combination of a diphenylamine, thiophosphate and phenyl-.alpha.-naphthylamine in a hydrocarbon lubricant base oil.